Such a scanning unit includes a scanning structure provided on a carrier and formed such that by interaction of an electromagnetic radiation (e.g., in the form of light), used for scanning the measuring graduation, both with the measuring graduation and with the scanning structure, a periodic stripe pattern is generated. The scanning unit further includes a detector arrangement for detecting that stripe pattern, which is made up of a plurality of detector elements of different allocated phases arranged one after another, the arrangement of detector elements of different phases forming a periodic pattern with respect to the phases. Each detector element is assigned exactly one partial area, referred to as a bar, of the scanning structure. The detector elements are furthermore combined to form two detector groups such that detector elements of the same phase belong to the same detector group. The detector elements combined to form a detector group are in each case interconnected for generating an output signal of the detector arrangement. This is accomplished regularly in the manner that, in each case detector elements of a first phase (e.g., of phase 0°) and the phase inverted to it (thus, of phase 180° in the example) are interconnected to form a first detector group. Detector elements of a further phase different therefrom (e.g., of phase 90°) and the phase inverted to it (thus, of phase 270° in the example) are interconnected to form a second detector group. The detector elements of a specific phase are interconnected differentially with detector elements of the phase inverted to it.
A periodic arrangement of detector elements should be understood not as an infinite period in the strict mathematical sense, but rather an arrangement of detector elements of different phase, such that the detector elements are arranged one after another with varying phase by in each case providing a detector element of every other phase exactly once between two detector elements of the same phase.
In European Published Patent Application No. 1 081 457, a position measuring device having a scanning unit of the type indicated at the outset is described, the detector arrangement of which has a plurality of detector elements of the phases 0°, 90°, 180° and 270° arranged one after another, the detector elements of the phases 0° and 180° and the detector elements of the phases 90° und 270° being interconnected at the output side. The arrangement of detector elements is periodic in terms of the assigned phases, i.e., a detector element of the phase 0° is always followed by a detector element of the phase 90°, after that, in turn, a detector element of the phase 180°, and finally a detector element of the phase 270°. Thereupon, this period repeated, beginning with a detector element of the phase 0°.
The phase information with respect to the detector elements relates in each case to the position of the respective detector element within a period formed by four detector elements, which is such that the output signals generated by adjacent detector elements each have a phase shift of 90° relative to each other. In this context, the detector elements having a phase shift of 180° relative to each other, which accordingly form the next neighbors but one in the detector arrangement, are interconnected differentially, so that their output signals are combined to form one unified output signal of the detector arrangement.
If such a scanning unit is used to scan a measuring graduation that extends along a curved path, for example, a measuring graduation applied on a drum having a defined radius, a phase-angle error may occur between the signals generated by the two detector groups, because the phase of the Vernier stripe pattern, which is generated by interaction of the electromagnetic radiation (light), used for the scanning, with the measuring graduation and the scanning structure, does not have an exact saw-tooth shape.